The global transport network has evolved to use SONET/SDH technology. However, benefits are gained by adding new capabilities to the previous generation asynchronous/plesiochronous digital hierarchy signal (e.g., DS1 and E1) technologies. Both the North American asynchronous hierarchy and the plesiochronous digital hierarchy (PDH) are referred to herein as PDH. These networks, however, are ubiquitously deployed and are more common than SONET/SDH signals for enterprise access applications. Among the reasons for their ongoing prevalence in the enterprise access networks is that many of the access interfaces are still delivered over copper wires.
At least as important as their availability is the advantage they provide due to the regulatory unbundling of services. U.S. incumbent local exchange carriers (ILECs) are required as part of unbundling to offer DS1 and DS3 access links to other carriers, such as competitive local exchange carriers (CLECs) or interexchange carriers (IECs), for lower tariff rates than equivalent SONET interfaces. The result of the tariff advantage and the effectively ubiquitous availability of DS1 and DS3 connectivity is that when IECs or service providers lack their own facilities to connect to their enterprise subscribers, they typically lease DS1 or DS3 connections through the ILECs. An example of such a network configuration is shown in FIG. 1.
The availability of PDH based networks combined with the growing interest in providing native Ethernet connectivity leads inevitably to a desire for mapping of Ethernet into PDH signals. Although a number of proprietary implementations exist, there are no standards for mapping native Ethernet into DS1 and DS3, or N×DS1 and N×DS3 signals. In order to provide Ethernet connectivity to their enterprise customers over DS1/DS3 connections, the major U.S. IECs have asked for GFP mappings into DSn and En signals.
GFP provides an encapsulation of native Ethernet frames in order to carry them through a transport network; see PMC white paper entitled “A tutorial on SONET/SDH”, PMC-2030895, the content of which is incorporated herein by reference in its entirety, and attached as Exhibit A. The resulting mappings are specified in the new ITU-T G.8040. Mapping data into multiple DSn or En signals is described in the following publicly available publications, the content of each of which is incorporated herein by reference in its entirety:                IETF RFC 1990, The PPP Multilink Protocol (MP). K. Sklower, B. Lloyd, G. McGregor, D. Carr, T. Coradetti. August 1996        Interoperability Requirements for N×56/64 kbit/s Calls, version 1.0, from the Bandwidth ON Demand INteroperability Group (BONDING) Consortium, 1992        ATM Forum AF-PHY-0086.001 (1999)—Inverse Multiplexing for ATM (IMA) Specification        
The GFP mapping into a single DS3 signal was defined first while mappings into DS1, E1, N×DS1, and N×E1 were studied. Carriers wanted to have the N×DS1 and N×E1 connections and use N=1 for mapping into single signals. Subsequently, interest developed for similar N×DS3 and N×E3 signals (e.g., for carrying data from 100Base Ethernet interfaces). Ideally, the N×DS1/E1/DS3/E3 should operate at Layer 1, providing transparent transport of Layer 2 protocol frames, independent of which Layer 2 protocol is being carried. The only existing non-proprietary solution is the Multilink Point-to-Point Protocol (ML-PPP defined in IETF RFC 1990), which performs inverse multiplexing at the packet level. Inverse multiplexing refers to taking the payload from a higher rate channel and transporting it by distributing it over multiple lower rate channels. The granularity used for assigning the payload data among the lower rate channels can be at the bit, byte, or packet/cell level.
Since ML-PPP is a Layer 2 protocol, it requires terminating the Ethernet signal in order to remap the packets into ML-PPP (i.e., change between the two different Layer 2 protocols). ATM solutions existed, including Inverse Multiplexing over ATM (IMA). The carriers requesting the new mapping did not favor an ATM solution for this application due to its overhead inefficiency and it being another layer to provision. No byte level inverse multiplexing schemes such as VCAT existed since DS1 and DS3 signals lacked sufficient overhead to support VCAT, and reserving an entire payload channel for the overhead was too much capacity to lose.
Another potential solution exist from the Bandwidth ON Demand Interoperability Group (BONDING) consortium. Inverse multiplexing here is performed at the byte level rather than the packet level. An initialization sequence is sent on all the constituent lower-rate channels in order to synchronize the source and sink. While this technique requires no per-packet or per-link overhead, the channel must be disrupted for a long period of re-initialization when the channel size is changed. Table 1 below shows a comparison of the different candidate technologies that were considered.
TABLE 1Comparison of technologies for inverse multiplexing into NxPDH signalsOPTIONADVANTAGEDISADVANTAGELayer 2 frameProven technology exists for ML-Layer 2 technology specific - ItinversePPP and Ethernet Linkeither enforces a Layer 2 approachmultiplexingAggregationor requires re-mapping client dataNo overhead required for eachpackets.individual E1/DS1/DS3/E3 linkRequires additional per-packetEasy to add or remove links (trivialoverhead (e.g., for packet sequencecontrol protocol)numbering)NOTE - Layer 1 (i.e., GFP) packetEgress queue management moreinterleaving was also considered, withcomplex due to the need to re-alignat least one proprietary solutionthe packets from the different linksexisting. Although it provides thein the correct sequence.Layer 2 transparency, it otherwise hasWhen there is a light load, a singlethe same advantages and disadvantageslink (or subset of links) is used foras Layer 2 packet interleaving.each packet rather than the entireset. This results in increased latencyfor lightly loaded cases.Under any load condition, the egressqueue management will tend tointroduce additional latency.Byte inverseRelatively simple.Changing the number of linksmultiplexingUses no additional per-link or per-(members) requires taking thewith overheadpacket overhead.connection down for a link re-borrowingsynchronization.Byte inverseSimple (trivial) egress buffer sinceRequires per-link overhead.multiplexingout-of-order packet arrival is notControl protocol for adding andwith permanentpossible.removing links is more complexoverheadCan directly re-use SDH virtual(same complexity as LCAS).channelconcatenation technology.No additional per-packet overhead.Consistency with VCAT andLCAS provides operationalconsistency and networkpredictability for the carrier.